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Mechanistic Insights into the C-H Bond Activation of Hydrocarbons by Chromium(IV) Oxo and Chromium(III) Superoxo Complexes
- Mechanistic Insights into the C-H Bond Activation of Hydrocarbons by Chromium(IV) Oxo and Chromium(III) Superoxo Complexes
- Cho, KB[Cho, Kyung-Bin]; Kang, H[Kang, Hyeona]; Woo, J[Woo, Jaeyoung]; Park, YJ[Park, Young Jun]; Seo, MS[Seo, Mi Sook]; Cho, J[Cho, Jaeheung]; Nam, W[Nam, Wonwoo]
- DGIST Authors
- Cho, J[Cho, Jaeheung]
- Issue Date
- Inorganic Chemistry, 53(1), 645-652
- Article Type
- Chemical Structure; Chemistry; Chromium; Conformation; Crystallography, X-Ray; Hydrocarbon; Hydrocarbons; Models, Molecular; Molecular Conformation; Organometallic Compound; Organometallic Compounds; Oxygen; Quantum Theory; X Ray Crystallography
- The mechanism of the C-H bond activation of hydrocarbons by a nonheme chromium(IV) oxo complex bearing an N-methylated tetraazamacrocyclic cyclam (TMC) ligand, [CrIV(O)(TMC)(Cl)]+ (2), has been investigated experimentally and theoretically. In experimental studies, reaction rates of 2 with substrates having weak C-H bonds were found to depend on the concentration and bond dissociation energies of the substrates. A large kinetic isotope effect value of 60 was determined in the oxidation of dihydroanthracene (DHA) and deuterated DHA by 2. These results led us to propose that the C-H bond activation reaction occurs via a H-atom abstraction mechanism, in which H-atom abstraction of substrates by 2 is the rate-determining step. In addition, formation of a chromium(III) hydroxo complex, [CrIII(OH)(TMC)(Cl)] + (3), was observed as a decomposed product of 2 in the C-H bond activation reaction. The CrIIIOH product was characterized unambiguously with various spectroscopic methods and X-ray crystallography. Density functional theory (DFT) calculations support the experimental observations that the C-H bond activation by 2 does not occur via the conventional H-atom-abstraction/oxygen-rebound mechanism and that 3 is the product formed in this C-H bond activation reaction. DFT calculations also propose that 2 may have some CrIIIO•- character. The oxidizing power of 2 was then compared with that of a chromium(III) superoxo complex bearing the identical TMC ligand, [CrIII(O 2)(TMC)(Cl)]+ (1), in the C-H bond activation reaction. By performing reactions of 1 and 2 with substrates under identical conditions, we were able to demonstrate that the reactivity of 2 is slightly greater than that of 1. DFT calculations again support this experimental observation, showing that the rate-limiting barrier for the reaction with 2 is slightly lower than that of 1. © 2013 American Chemical Society.
- American Chemical Society
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